We propose to study the developmental parameters which guide the early growth and determination of the head of Drosophila melanogaster. Clonal analysis will be accomplished through use of genetic cell markers and X-ray induced mitotic recombination. The developmental restriction, imposed on head anlage cells are detected by use of clones whose cells have an autonomously higher division rate than that in neighboring cells. These restrictions lead to a developmental compartmentalization of contiguous cells beginning shortly after blastoderm and continuing in a sequential manner for several cell divisions. A detailed fate map of the blastoderm head cells has aided in the establishment of these compartments. Further analysis of head development will be undertaken by: (1) employing an opthalmoptera homeotic mutant in a clonal analysis to relate directly dorsal and ventral in head and wing, (2) studying by clonal analysis a mutant which produces extra compound eyes in the dorsal-posterior head region, (3) learning the relation between the developmental border between dorsal and ventral compartments in the eye and the eye equator formed by mirror image symmetry of the photoreceptor cells in the upper and lower half of this organ, (4) studying at later stages the developmental compartmentalization of the head coupled with more detailed fate mapping of specific head regions, (5) making a clonal analysis as well as a detailed gynandromorph fate map of the antenna.